Extend tests for opendoors/external_authors_controller

opendoors/external_authors_controller currently has 28% test coverage on Coveralls:
hhttps://coveralls.io/builds/9457218/source?filename=app%2Fcontrollers%2Fopendoors%2Fexternal_authors_controller.rb

We'd like to increase that, with an ultimate goal of 90% or greater.

How to test:
Use Coveralls to confirm that the coverage percentage has increased:
1. Go to https://coveralls.io/github/otwcode/otwarchive?branch=master
2. In the "Search" field near the "Source Files on Master" heading, enter the name of the controller: opendoors/external_authors_controller
3. Note the number in the "Coverage" column of the table